Download Ch_5

Chapter 5:
NEWTON’S THIRD LAW OF
MOTION
© 2010 Pearson Education, Inc.
This lecture will help you understand:
•
•
•
•
Forces and Interactions
Newton’s Third Law of Motion
Summary of Newton’s Laws
Vectors
© 2010 Pearson Education, Inc.
Forces and Interactions
Interaction
• is between one thing and another.
• requires a pair of forces acting on two objects.
Example: interaction of hand and
wall pushing on each other
Force pair—you push on wall; wall
pushes on you.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR NEIGHBOR
A soccer player kicks a ball with 1500 N of force. The ball
exerts a reaction force against the player’s foot of
A.
B.
C.
D.
somewhat less than 1500 N.
1500 N.
somewhat more than 1500 N.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR ANSWER
A soccer player kicks a ball with 1500 N of force. The ball
exerts a reaction force against the player’s foot of
A.
B.
C.
D.
somewhat less than 1500 N.
1500 N.
somewhat more than 1500 N.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
Action and reaction forces
• one force is called the action force; the other
force is called the reaction force.
• are co-pairs of a single interaction.
• neither force exists without the other.
• are equal in strength and opposite in direction.
• always act on different objects.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
• Reexpression of Newton’s third law: To every
action there is always an opposed equal
reaction.
Example: Tires of car push back against the road while
the road pushes the tires forward.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
Simple rule to identify action and reaction
• Identify the interaction—one thing interacts with
another
– Action: Object A exerts a force on object B.
– Reaction: Object B exerts a force on object A.
Example: Action—rocket (object A) exerts force on
gas (object B).
Reaction—gas (object B) exerts force on
rocket (object A).
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR NEIGHBOR
When you step off a curb, Earth pulls you downward. The
reaction to this force is
A.
B.
C.
D.
a slight air resistance.
nonexistent in this case.
you pulling Earth upward.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR ANSWER
When you step off a curb, Earth pulls you downward. The
reaction to this force is
A.
B.
C.
D.
a slight air resistance.
nonexistent in this case.
you pulling Earth upward.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR NEIGHBOR
When you step off a curb, Earth pulls you downward and
you pull the force upward. Why do you not sense Earth
moving upward toward you?
A.
B.
C.
D.
Earth is fixed, so it cannot move.
Earth can move, but other objects on it prevent it from
moving.
It moves, but a very small amount that you cannot see.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR ANSWER
When you step off a curb, Earth pulls you downward and
you pull the force upward. Why do you not sense Earth
moving upward toward you?
A.
B.
C.
D.
Earth is fixed, so it cannot move.
Earth can move, but other objects on it prevent it from
moving.
It moves, but a very small amount that you cannot see.
None of the above.
Explanation:
You exert a force on Earth that is equal to the force it
exerts on you. But you move more than the Earth does,
because its mass is so great compared to your mass that
it moves very little and you do not notice it.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
Action and Reaction on Different Masses
Cannonball: F
=a
m
Cannon: F = a
m
• The same force exerted on a small mass
produces a large acceleration.
• The same force exerted on a large mass
produces a small acceleration.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR NEIGHBOR
When a cannon is fired, the accelerations of the cannon
and cannonball are different because the
A.
B.
C.
D.
forces don’t occur at the same time.
forces, although theoretically the same, in practice are
not.
masses are different.
ratios of force to mass are the same.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR ANSWER
When a cannon is fired, the accelerations of the cannon
and cannonball are different because the
A.
B.
C.
D.
forces don’t occur at the same time.
forces, although theoretically the same, in practice are
not.
masses are different.
ratios of force to mass are the same.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR NEIGHBOR
Consider a high-speed bus colliding head-on with an
innocent bug. The force of impact splatters the unfortunate
bug over the windshield.
Which is greater, the force on the bug or the force on the
bus?
A.
B.
C.
D.
Bug
Bus
Both are the same.
Cannot say
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR ANSWER
Consider a high-speed bus colliding head-on with an
innocent bug. The force of impact splatters the unfortunate
bug over the windshield.
Which is greater, the force on the bug or the force on the
bus?
A.
B.
C.
D.
Bug
Bus
Both are the same.
Cannot say
© 2010 Pearson Education, Inc.
Comment:
Although the forces are equal
in magnitude, the effects are
very different. Do you know
why?
Newton’s Third Law of Motion
CHECK YOUR NEIGHBOR
Two people of equal mass on slippery ice push off from
each other. Will both move at the same speed in opposite
directions?
A.
B.
C.
D.
Yes
Yes, but only if both push equally
No
No, unless acceleration occurs
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
CHECK YOUR ANSWER
Two people of equal mass on slippery ice push off from each
other. Will both move at the same speed in opposite directions?
A.
B.
C.
D.
Yes
Yes, but only if both push equally
No
No, unless acceleration occurs
Explanation:
However they push, the result is equal-magnitude forces on equal
masses, which produces equal accelerations; therefore, there are
equal changes in speed.
© 2010 Pearson Education, Inc.
Since action and reaction forces
are equal and opposite, why
don’t they cancel to zero?
Defining Your System
• Consider a single enclosed
orange.
– Applied external force
causes the orange to
accelerate in accord with
Newton’s second law.
– Action and reaction pair of
forces is not shown.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
• Consider the orange and the apple pulling on it.
– Action and reaction do not cancel (because
they act on different things).
– External force by apple accelerates the
orange.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
• Consider a system comprised of both the orange
and the apple
– The apple is no longer external to the system.
– Force pair is internal to system, which doesn’t cause
acceleration.
– Action and reaction within the system cancel.
– With no external forces, there is no acceleration of
system.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
• Consider the same system, but with external
force of friction on it.
– Same internal action and reaction forces
(between the orange and apple) cancel.
– A second pair of action-reaction forces
(between the apple’s feet and the floor) exists.
© 2010 Pearson Education, Inc.
Newton’s Third Law of Motion
– One of these acts by the system (apple on the
floor) and the other acts on the system (floor
on the apple).
– External frictional force of floor pushes on the
system, which accelerates.
– Second pair of action and reaction forces do
not cancel.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR NEIGHBOR
When lift equals the weight of a helicopter, the helicopter
A.
B.
C.
D.
climbs down.
climbs up.
hovers in midair.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR ANSWER
When lift equals the weight of a helicopter, the helicopter
A.
B.
C.
D.
climbs down.
climbs up.
hovers in midair.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR NEIGHBOR
When lift is greater, the helicopter
A.
B.
C.
D.
climbs down.
climbs up.
hovers in midair.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR ANSWER
When lift is greater, the helicopter
A.
B.
C.
D.
climbs down.
climbs up.
hovers in midair.
None of the above.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR NEIGHBOR
A bird flies by
A.
B.
C.
D.
flapping its wings.
pushing air down so that the air pushes it upward.
hovering in midair.
inhaling and exhaling air.
© 2010 Pearson Education, Inc.
Newton’s Third Law
CHECK YOUR ANSWER
A birds flies by
A.
B.
C.
D.
flapping its wings.
pushing air down so that the air pushes it upward.
hovering in midair.
inhaling and exhaling air.
© 2010 Pearson Education, Inc.
Summary of Newton’s Three
Laws of Motion
• Newton’s first law of motion (the law of inertia)
– An object at rest tends to remain at rest; an object in motion
tends to remain in motion at constant speed along a straight-line
path.
• Newton’s second law of motion (the law of acceleration)
– When a net force acts on an object, the object will accelerate.
The acceleration is directly proportional to the net force and
inversely proportional to the mass.
• Newton’s third law of motion (the law of action and
reaction)
– Whenever one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first.
© 2010 Pearson Education, Inc.
Vectors
Vector quantity
• has magnitude and direction.
• is represented by an arrow.
Example: velocity, force, acceleration
Scalar quantity
• has magnitude.
Example: mass, volume, speed
© 2010 Pearson Education, Inc.
Vectors
Resultant
• The sum of two or more vectors
– For vectors in the same direction, add arithmetically.
– For vectors in opposite directions, subtract
arithmetically.
– Two vectors that don’t act in the same or opposite
direction:
• use parallelogram rule.
– Two vectors at right angles to each other
• use Pythagorean Theorem: R2 = V2 + H2.
© 2010 Pearson Education, Inc.
Vectors
CHECK YOUR NEIGHBOR
Referring to the figure, which of the following are true
statements?
A.
B.
C.
D.
50 N is the resultant of the 30- and 40-N vectors.
The 30-N vector can be considered a component of
the 50-N vector.
The 40-N vector can be considered a component of
the 50-N vector.
All of the above are correct.
© 2010 Pearson Education, Inc.
Vectors
CHECK YOUR ANSWER
Referring to the figure, which of the following are true
statements?
A.
B.
C.
D.
50 N is the resultant of the 30- and the 40-N vectors.
The 30-N vector can be considered a component of
the 50-N vector.
The 40-N vector can be considered a component of
the 50-N vector.
All of the above are correct.
© 2010 Pearson Education, Inc.
Vectors
CHECK YOUR NEIGHBOR
Referring to the figure, which of the following are true
statements?
A.
B.
C.
D.
100 km/h is the resultant of the 80- and 60-km/h vectors.
The 80-km/h vector can be considered a component of
the 100-km/h vector.
The 60-km/h vector can be considered a component of
the 100-km/h vector.
All of the above are correct.
© 2010 Pearson Education, Inc.
Vectors
CHECK YOUR ANSWER
Referring to the figure, which of the following are true
statements?
A.
B.
C.
D.
100 km/h is the resultant of the 80- and 60-km/h vectors.
The 80-km/h vector can be considered a component of the
100-km/h vector.
The 60-km/h vector can be considered a component of the
100-km/h vector.
All of the above are correct.
© 2010 Pearson Education, Inc.
Vectors
Vector components
• Vertical and horizontal components of a
vector are perpendicular to each othe.r
• Determined by resolution.
© 2010 Pearson Education, Inc.
Vectors
CHECK YOUR NEIGHBOR
You run horizontally at 4 m/s in a vertically falling rain that
falls at 4 m/s. Relative to you, the raindrops are falling at an
angle of
A.
B.
C.
D.
0.
45.
53.
90.
© 2010 Pearson Education, Inc.
Vectors
CHECK YOUR ANSWER
You run horizontally at 4 m/s in a vertically falling rain that
falls at 4 m/s. Relative to you, the raindrops are falling at an
angle of
A.
B.
C.
D.
0.
45.
53.
90.
Explanation:
The horizontal 4 m/s and vertical 4 m/s combine by the
parallelogram rule to produce a resultant of 5.6 m/s at
45.
© 2010 Pearson Education, Inc.
Vectors
Nellie Newton hangs from a
rope as shown.
• Which side has the greater
tension?
• There are three forces
acting on Nellie:
– her weight,
– a tension in the left-hand side
of the rope,
– and a tension in the righthand side of the rope.
© 2010 Pearson Education, Inc.
Vectors
• Because of the different angles, different rope tensions will
occur in each side.
• Nellie hangs in equilibrium, so her weight is supported by two
rope tensions, adding vectorially to be equal and opposite to
her weight.
• The parallelogram rule shows that the tension in the righthand rope is greater than the tension in the left-hand rope.
© 2010 Pearson Education, Inc.
From Conceptual Physics Textbook:
• Pg 77- 3, 4, 8, 11,15, 16, 22
• Pg 79 – 5, 6, 13, 27, 31, 32, 33, 34, 35
• Problems 2, 4, 6
• From Problem solving book (Ch 5):
• Practice the sample problems 1, 2, 3
• Pg 54 – 1, 2, 14, 24, 27, 28
© 2010 Pearson Education, Inc.